VAM is an important monomer building block in the synthesis of plastic polymers. The main fields of use of VAM are i.a. the preparation of polyvinyl acetate, polyvinyl alcohol and polyvinyl acetal and co- and terpolymerization with other monomers such as for example ethylene, vinyl chloride, acrylate, maleinate, fumarate and vinyl laurate.
VAM is produced predominantly in the gas phase from acetic acid and ethylene by reaction with oxygen, wherein the catalysts used for this synthesis preferably contain Pd and Au as active metals and also an alkali metal component as promoter, preferably potassium in the form of the acetate. In the Pd/Au system of these catalysts, the active metals Pd and Au are not present in the form of metal particles of the respective pure metal, but rather in the form of Pd/Au-alloy particles of possibly different composition, although the presence of unalloyed particles cannot be ruled out. As an alternative to Au, for example Cd or Ba can also be used as second active metal component.
Currently, VAM is predominantly produced by means of so-called shell catalysts in which the catalytic active metals of the catalyst do not fully penetrate the catalyst support formed as a shaped body, but rather are contained only in an outer area (shell) of greater or lesser width of the catalyst support shaped body (cf. on this EP 565 952 A1, EP 634 214 A1, EP 634 209 A1 and EP 634 208 A1), while the areas of the support lying further inside are almost free of noble metals. With the help of shell catalysts, a more selective reaction control is possible in many cases than with catalysts in which the supports are impregnated with the active components into the core of the support (“impregnated through”).
The shell catalysts known in the state of the art for the production of VAM can be for example catalyst supports based on silicon oxide, aluminium oxide, aluminosilicate, titanium oxide or zirconium oxide (cf. on this EP 839 793 A1, WO 1998/018553 A1, WO 2000/058008 A1 and WO 2005/061107 A1). Catalyst supports based on titanium oxide or zirconium oxide are currently scarcely used, however, as these catalyst supports do not display long-term stability compared with acetic acid and are relatively expensive.
The great majority of the catalysts currently used for the production of VAM are shell catalysts with a Pd/Au shell on a porous amorphous aluminosilicate support formed as a sphere based on natural sheet silicates in the form of natural acid-treated calcined bentonites which are thoroughly impregnated with potassium acetate as promoter.
Such VAM shell catalysts are usually produced by the so-called chemical route in which the catalyst support is loaded with or soaked in solutions of corresponding metal precursor compounds, for example by dipping the support into the solutions, or by means of the incipient wetness method (pore-filling method) in which the support is loaded with or soaked in a volume of solution corresponding to its pore volume. The Pd/Au shell of the catalyst is produced for example by first soaking the catalyst support shaped body in a first step in an Na2PdCl4 solution and then in a second step fixing the Pd component with NaOH solution onto the catalyst support in the form of a Pd-hydroxide compound. In a subsequent, separate third step, the catalyst support is then soaked in an NaAuCl4 solution and then the Au component is likewise fixed by means of NaOH. After the fixing of the noble-metal components in an outer shell of the catalyst support, the loaded catalyst support is then very largely washed free of chloride and Na ions, then dried, calcined and finally reduced with ethylene at 150° C. The thus-produced Pd/Au shell is usually approximately 100 to 500 μm thick.
Usually, the catalyst support loaded with the noble metals is loaded with potassium acetate after the fixing or reducing step, wherein, rather than the loading with potassium acetate taking place only in the outer shell loaded with noble metals, the catalyst support is completely impregnated with the promoter. A spherical support called “KA-160” from SÜD-Chemie AG based on natural acid-treated calcined bentonites as sheet silicate, which has a BET surface area of approximately 160 m2/g, is predominantly used as catalyst support.
The VAM selectivities, achieved by means of the VAM shell catalysts known in the state of the art based on Pd and Au as active metals and KA-160 supports as catalyst supports, are approximately 90 mol-% relative to the supplied ethylene, wherein the remaining 10 mol-% of the reaction products are essentially CO2 which is formed by total oxidation of the organic educts/products.
To increase the activity of these catalysts, the active-metal-free catalyst support shaped bodies based on natural silicates were firstly surface-doped with zirconium oxide before deposition of the noble metal. For this, for example, a finished bentonite-based shaped body was impregnated with a solution of a zirconium oxide precursor compound and the precursor compound converted into the corresponding oxide by calcining of the shaped body.
Although, compared with the corresponding catalysts with a Pd/Au shell known in the state of the art, such catalysts are characterized by an increased activity in respect of VAM production, the activity can be increased to only a limited extent, as the ZrO2 coats pores in which Pd and Au of oxidation stage 0 are also to be deposited. If the support is overloaded with ZrO2, a decrease in the activity of the catalyst can be observed.